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Astronomy& Astrophysicsmanuscript no. aa_1ES1011_arxiv c©ESO 2019December 19, 2019

Insights into the emission of the blazar 1ES 1011+496 throughunprecedented broadband observations during 2011 and 2012

J. Aleksic1, S. Ansoldi2, L. A. Antonelli3, P. Antoranz4, A. Babic5, P. Bangale6, U. Barres de Almeida6,31,J. A. Barrio7, J. Becerra González8,25, W. Bednarek9, E. Bernardini10, B. Biasuzzi2, A. Biland11, O. Blanch1,

S. Bonnefoy7, G. Bonnoli3, F. Borracci6, T. Bretz12,26, E. Carmona13, A. Carosi3, P. Colin6, E. Colombo8,J. L. Contreras7, J. Cortina1, S. Covino3, P. Da Vela4, F. Dazzi6, A. De Angelis2, G. De Caneva10, B. De Lotto2, E. de

Oña Wilhelmi14, C. Delgado Mendez13, F. Di Pierro3, D. Dominis Prester5, D. Dorner12, M. Doro15, S. Einecke16,D. Eisenacher12, D. Elsaesser12, A. Fernández-Barral1, D. Fidalgo7, M. V. Fonseca7, L. Font17, K. Frantzen16,

C. Fruck6, D. Galindo18, R. J. García López8, M. Garczarczyk10, D. Garrido Terrats17, M. Gaug17, N. Godinovic5,A. González Muñoz1, S. R. Gozzini10, D. Hadasch14,27, Y. Hanabata19, M. Hayashida19, J. Herrera8, J. Hose6,

D. Hrupec5, W. Idec9, V. Kadenius, H. Kellermann6, M. L. Knoetig11, K. Kodani19, Y. Konno19, J. Krause6, H. Kubo19,J. Kushida19, A. La Barbera3, D. Lelas5, N. Lewandowska12, E. Lindfors20,28, S. Lombardi3, F. Longo2, M. López7,

R. López-Coto1, A. López-Oramas1, E. Lorenz6, I. Lozano7, M. Makariev21, K. Mallot10, G. Maneva21,K. Mannheim12, L. Maraschi3, B. Marcote18, M. Mariotti15, M. Martínez1, D. Mazin6, U. Menzel6, J. M. Miranda4,

R. Mirzoyan6, A. Moralejo1, P. Munar-Adrover18, D. Nakajima19, V. Neustroev20, A. Niedzwiecki9, M. NievasRosillo7, K. Nilsson20,28, K. Nishijima19, K. Noda6, R. Orito19, A. Overkemping16, S. Paiano15, M. Palatiello2,

D. Paneque6, R. Paoletti4, J. M. Paredes18, X. Paredes-Fortuny18, M. Persic2,29, J. Poutanen20, P. G. Prada Moroni22,E. Prandini11,30, I. Puljak5, R. Reinthal20, W. Rhode16, M. Ribó18, J. Rico1, J. Rodriguez Garcia6, T. Saito19, K. Saito19,

K. Satalecka7, V. Scalzotto15, V. Scapin7, C. Schultz15, T. Schweizer6, S. N. Shore22, A. Sillanpää20, J. Sitarek1,I. Snidaric5, D. Sobczynska9, A. Stamerra3, T. Steinbring12, M. Strzys6, L. Takalo20, H. Takami19, F. Tavecchio3,

P. Temnikov21, T. Terzic5, D. Tescaro8, M. Teshima6,19, J. Thaele16, D. F. Torres23, T. Toyama6, A. Treves24,P. Vogler11, M. Will 8, R. Zanin18, S. Buson15, F. D’Ammando32, A. Lähteenmäki33,34, T. Hovatta35,33,

Y. Y. Kovalev36,37, M. L. Lister38, W. Max-Moerbeck39, C. Mundell40, A. B. Pushkarev4142,37, E. Rastorgueva-Foi33,A. C. S. Readhead35, J. L. Richards37, J. Tammi33, D. A. Sanchez43, M. Tornikoski33, T. Savolainen37, and I. Steele40

(Affiliations can be found after the references)

Received August 12, 2015; accepted February 10, 2016

ABSTRACT

Context. 1ES 1011+496 (z = 0.212) was discovered in very high energy (VHE, E>100 GeV)γ-rays with MAGIC in 2007. Theabsence of simultaneous data at lower energies led to a rather incomplete characterization of the broadband spectral energy distribution(SED).Aims. We study the source properties and the emission mechanisms,probing whether a simple one-zone synchrotron-self-Compton(SSC) scenario is able to explain the observed broadband spectrum.Methods. We analyzed VHE to radio data from 2011 and 2012 collected by MAGIC, Fermi-LAT, Swift, KVA, OVRO, and Metsähoviin addition to optical polarimetry data and radio maps from the Liverpool Telescope and MOJAVE.Results. The VHE spectrum was fit with a simple power law with a photon index of 3.69 ± 0.22 and a flux above 150 GeV of(1.46± 0.16)× 10−11 ph cm−2 s−1. 1ES 1011+496 was found to be in a generally quiescent state at all observed wavelengths, showingonly moderate variability from radio to X-rays. A low degreeof polarization of less than 10% was measured in optical, while somebright features polarized up to 60% were observed in the radio jet. A similar trend in the rotation of the electric vector position anglewas found in optical and radio. The radio maps indicated a superluminal motion of 1.8± 0.4c, which is the highest speed statisticallysignificantly measured so far in a high-frequency-peaked BLLac.Conclusions. For the first time, the high-energy bump in the broadband SED of 1ES 1011+496 could be fully characterized from0.1 GeV to 1 TeV which permitted a more reliable interpretation within the one-zone SSC scenario. The polarimetry data suggest thatat least part of the optical emission has its origin in some ofthe bright radio features, while the low polarization in optical might bedue to the contribution of parts of the radio jet with different orientations of the magnetic field to the optical emission.

Key words. BL Lacertae objects: individual: 1ES 1011+496 - galaxies: active - galaxies: jets - gamma rays: galaxies - radiationmechanisms: non-thermal

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1. Introduction

Blazars are a subclass of radio-loud active galactic nuclei(AGNs) with their relativistic particle jets closely aligned tothe line of sight of the observer. They are highly variable atnearly all wavelengths at various timescales, and their emissionis dominated by a non-thermal continuum spanning from radioto VHE γ-rays which is assumed to be produced within the jetsand boosted by beaming (e.g.Urry & Padovani 1995; Ghisellini2000). The spectral energy distribution (SED) of blazars showstwo distinct broad components: a low-energy bump in the opticalto X-rays range that is commonly associated with synchrotronemission of electrons, and a high-energy bump in theγ-ray band.The origin of the latter component is usually explained by lep-tonic models in terms of IC scattering of synchrotron photons(e.g.Tavecchio et al. 1998; Katarzynski et al. 2001) or externalphotons (e.g.Sikora et al. 1994), but hadronic emission models(e.g.Mannheim 1993), have also been proposed.

Based on their optical spectra (e.g.Stickel 1991), blazarsare divided into two classes: flat spectrum radio quasars (FS-RQs) that show broad emission lines, and BL Lac objects char-acterized by the weakness or even absence of such lines. Thelatter were further subdivided into low- and high-energy cut-off BL Lacs (LBLs, HBLs) depending on the radio-to-X-rayspectral slope, which reflects into the SED’s synchrotron peakposition (Padovani & Giommi 1995; Urry & Padovani 1995).An alternative definition was given inAbdo et al. (2010a),where blazars are classified as low-, intermediate-, and highsynchrotron-peaked blazars (LSP, ISP, HSP) based on the loca-tion of the synchrotron peak. Later on,Spurio (2014) definedLBLs, IBLs and HBLs according to the synchrotron peak po-sitions given inAbdo et al.(2010a) for LSPs, ISPs and HSPs.Since blazars show flux variability at all wavelengths at differenttimescales ranging down to minutes, simultaneous observationsare a useful tool to study the overall SED and to constrain thephysical processes that govern the emission in their jets.

1ES 1011+496 (RA = 10:15:04.14, Dec= 49:26:00.70;J2000) is a blazar located at redshift ofz = 0.212 ±0.002 (Albert et al. 2007a)1 classified as an HBL based on theradio-to-X-ray ratio (Padovani & Giommi 1995; Donato et al.2001) and the presence of a featureless optical spec-trum (Wisniewski et al. 1986). It was suggested as VHEγ-ray candidate with a predicted integral flux of 0.12 ×10−11 ph cm−2 s−1 above 300 GeV byCostamante & Ghisellini(2002). From 1996 to 2006 the source was target of severalVHE γ-ray observations by HEGRA (Aharonian et al. 2004), theWhipple Observatory 10 mγ-ray telescope (Fegan et al. 2005)and MAGIC (Albert et al. 2008a; Aleksic et al. 2011) yieldingintegral flux upper limits only. In 2007 MAGIC detected thesource first in the VHE regime (Albert et al. 2007a) and sub-sequently detected it in 2008 (Ahnen et al. 2015). Consider-ing the first two years ofFermi-LAT observations reported inthe secondFermi-LAT catalog (2FGL;Nolan et al. 2012), 1ES1011+496 is associated with the source 2FGL J1015.1+4925,which has been observed with an integral flux of (4.4 ± 0.3) ×10−8 ph cm−2 s−1 (100 MeV−100 GeV). The high-energy (HE,100 MeV < E < 100 GeV)γ-ray spectrum could be fit with

a log parabola of the formdNdE = N0

(

EEb

)−(α+β log(E/Eb)), where

N0 = (1.01±0.04)×10−11ph cm−2 s−1 MeV−1 andα = 1.72±0.04denote the normalized flux and the spectral index, respectively,

1This redshift corresponds to a luminosity distance of 1.04 Gpc forcontemporary cosmology parameters, i.e.H0 = 71 km s−1 Mpc−1,ΩΛ =0.73,Ωc = 0.27 (Spergel et al. 2003).

at the pivot energyEb = 812.6 MeV andβ = 0.075± 0.019is a measure of the spectral curvature. In the thirdFermi-LATsource catalog (3FGL;Acero et al. 2015) an integral flux of(5.1±0.2)×10−8 ph cm−2 s−1 (100 MeV−100 GeV) was reported.A simple power-law fit with a photon index of 1.83± 0.02 wassufficient to describe the spectrum obtained from 4 years ofFermi operation. Above 10 GeV the spectrum is well describedby a simple power-law fit with a photon index of 2.28±0.16 andthe integral flux corresponds to (7.87± 0.89)× 10−8 ph cm−2 s−1

(10−500GeV) as reported in the firstFermi High-energy LATcatalog (1FHL,E > 10 GeV;Ackermann et al. 2013).

Based on archival MWL data,Ahnen et al.(2015) discussthat the source’s characteristics resemble those of an IBL duringlow-to-medium flux states, whereas at high states they are simi-lar to an HBL, concluding therefore that the source seems to bea boderline case between IBL and HBL.

In blazar studies the polarization represents a powerful toolto distinguish between the competing physical models regardingthe particle and seed photon populations responsible for theirVHE γ-ray emission (e.g.Pavlidou et al. 2013). Furthermore,the study of the position angle provides information on the ori-entation of the magnetic field of the emission region thus helpingto understand the state of the plasma and the particle populationin the location of emission (e.g.Barres de Almeida et al. 2010).In some cases, large changes in polarization angle have beenas-sociated withγ-ray flares (e.g.Abdo et al. 2010b), but the linkbetween rotations in polarization angle and high-energy activityis still under study (e.g.Blinov et al. 2015).

In this paper we report for the first time MAGIC stereoobservations of 1ES 1011+496, carried out from 2011 to2012, and provide a more accurate VHEγ-ray spectrum(Sect.3.1) than those measured in 2007 (Albert et al. 2007a) and2008 (Ahnen et al. 2015), when MAGIC operated with a singletelescope. We discuss the multiwavelength (MWL) variability(Sect.3.2) of the source based on simultaneous data in HEγ-rays fromFermi Large Area Telescope (LAT), in X-rays andUV bands bySwift (XRT/UVOT), in the optical R-band by theKVA telescope and in the radio band at 37 and 15 GHz by theMetsähovi and OVRO telescopes respectively. The individual in-struments involved in these MWL observations are describedinSect.2 including information on the observations and the dataanalysis. We combine these MWL observations with optical po-larimetry data from the Liverpool Telescope and multi-epochradio maps from MOJAVE2 in order to put further constraintson the site and structure of the VHEγ-ray emission region. Wemodel the broad band SED compiled from these MWL observa-tions assuming a one-zone SSC scenario (Sect.4). Our conclu-sions are summarized in Sect.5.

2. Observation and data analysis

2.1. MAGIC

Since 2009 MAGIC is operating as a stereoscopic system of two17 m Imaging Atmospheric Cherenkov Telescopes, MAGIC Iand MAGIC II, that are located at the Roque de Los Muchachos,La Palma, Canary Islands (28.8 N, 17.9 W, 2225 m a.s.l.). Dueto its low energy threshold (as low as 60 GeV in normal triggermode) and high sensitivity3, MAGIC is a well suited instrumentfor VHE γ-ray observations of blazars. From summer 2011 to

2Monitoring of Jets in Active Galactic Nuclei with VLBA Experi-ments (Lister et al. 2009)

3Better than 0.8% of the Crab Nebula flux in 50 h of observingtime above 290 GeV (Aleksic et al. 2012) in stereoscopic mode, while

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Aleksic et al.: MAGIC and multifrequency observations of 1ES 1011+496 in 2011 and 2012

summer 2012, a further upgrade of the MAGIC was carried outby decommissioning the old MAGIC I camera and MUX read-out electronics with the aim to further improve the performanceof the MAGIC stereo system that has been limited by the smallertrigger region and the slightly lower light conversion efficiencyof the MAGIC I camera (Mazin et al. 2013).

In a first phase in late 2011, the readout system of theMAGIC I telescope was upgraded to a digitizing system basedon the domino-ring-sampler (DRS4) version 4. Compared to theDRS-2 chip, previously used in MAGIC II, the dead time hasbeen significantly reduced to less than 1% (Sitarek et al. 2013).In the course of this hardware change, the MAGIC II readoutsystem was also updated to this latest chip version. The dom-inant sources of systematic uncertainties are not related to thereadout system, but rather the spectral reflectivity of the mirrors,the camera photon detection efficiency and the atmospheric char-acterisation; and hence the prescription reported inAleksic et al.(2012) is still valid for the 2012 data

1ES 1011+496 was observed with MAGIC during darknights and under moderate Moon conditions at zenith anglesspanning from 24 to 50. In 2011, observations were performedduring 12 individual nights between the end of February and be-ginning of April for a total of∼13 hours, while observations in2012 were performed from the end of January until mid of Mayduring 33 nights for a total of∼23 hours with the upgraded read-out system.

The total effective observation time after corrections forthe dead time of the readout system is∼30.6 hours. Observa-tions were performed in the so-calledwobble mode (Fomin et al.1994) during which both telescopes alternated every 20 minutesbetween two (in 2011), respectively four (in 2012), sky posi-tions with an offset of 0.4 from the source. The data were an-alyzed using the MAGIC analysis and reconstruction software(MARS) package (Zanin et al. 2013) that has been adapted tostereoscopic observations. The image cleaning was performedaccording toAliu et al. (2009).

The images were parametrized in each telescope individu-ally according to the prescription ofHillas (1985). For the re-construction of the shower arrival direction the random forest re-gression method (RF DISP method;Aleksic et al. 2010) with theimplementation of stereoscopic parameters such as the impactdistance of the shower on the ground was used (Lombardi et al.2011). Theγ/hadron separation was performed by using the ran-dom forest method (Albert et al. 2008c) which is based on bothindividual image parameters from each telescope and stereo-scopic information such as the shower impact point and theshower height maximum. Energy look-up tables were used forthe energy reconstruction. Further details on the stereo MAGICanalysis can be found inAleksic et al.(2012).

For sources with VHEγ-ray spectra similar to that of theCrab Nebula, the sensitivity of the MAGIC stereo system is bestabove 250 – 300 GeV. For sources with spectral shapes softerthan that of the Crab Nebula, the best performance occurs atslightly lower energies. Consequently, we chose 150 GeV as theminimum energy to report signal significances andγ-ray fluxesin light curves, while for the spectral analysis, in order touse allthe available information, we also considered energies well be-low 150 GeV, where the analysis of the MAGIC data can still beperformed (Aleksic et al. 2016).

in mono mode the best sensitivity achieved above 250 GeV was 2.2%of the Crab Nebula flux in 50 h (Albert et al. 2008b).

4http://www.psi.ch/drs/

2.2. Fermi-LAT

1ES 1011+496 has been observed by the pair-conversion tele-scopeFermi-LAT optimized for energies from 20 MeV up to en-ergies beyond 300 GeV (Atwood et al. 2009). In survey modetheFermi-LAT scans the entire sky every three hours. The datasample, which consists of observations from 2011 February 24to April 7, and from 2012 January 1 to May 30, was analyzedwith the standard analysis toolgtlike, part of theFermi Sci-ence Tools software package (version 09-27-01) available fromthe Fermi Science Support Center.5 We selected events of theCLEAN6class with with energies from 100 MeV to 300 GeV lo-cated in a circular region of interest (ROI) of 10 radius centeredon the position of 1ES 1011+496. Time intervals when the LATboresight was rocked with respect to the local zenith by morethan 52 and events with a reconstructed angle with respect tothe local zenith> 100 were excluded. The latter selection wasnecessary to limit the contamination fromγ-rays produced byinteractions of cosmic rays with the upper atmosphere of theEarth. In addition, to correct the calculation of the exposure forthe zenith cut, time intervals when any part of the ROI was ob-served at zenith angles> 100 were excluded. For theγ-ray sig-nal extraction, the background model included two components:a Galactic diffuse emission and an isotropic diffuse, providedby the publicly available files gal_2yearp7v6_trim_v0.fitsandiso_p7v6clean.txt.7 The model of the ROI also included sourcesfrom the 2FGL (Nolan et al. 2012) that are located within 15

of 1ES 1011+496. These sources, as well as the source of in-terest, were modeled with a power-law spectral shape. We firstfitted the whole dataset considered in this paper and then usedthe resulting best-fit ROI model to produce the light curve andSED. In the light curve and SED fitting, the spectral parametersof sources within 10 from our target were allowed to vary whilethose within10 − 15 were fixed to their initial values. Duringthe spectral fitting, the normalizations of the background modelswere allowed to vary freely. Spectral parameters were estimatedfrom 300 MeV to 300 GeV using an unbinned maximum likeli-hood technique (Mattox et al. 1996) taking into account the post-launch instrument response functions (specifically P7CLEAN_-V6, Ackermann et al. 2012). When producing the SED and thelight curves only the parameter of the source of interest werefree to vary. The parameters of other sources in the ROI werekept fixed to average values found over the studied period.

During the MAGIC observing period, the source was notsignificantly detected on a daily basis. To ensure a good com-promise between having a significant detection in most of theintervals and details on the temporal behavior of the source, thelight curves were produced with weekly binning for the 2011period, and with a 3-day binning for the 2012 period (secondpanel from the top in Fig.3). To produce theFermi-LAT SED,simultaneous to the MAGIC observation periods, the aforemen-tioned 2011 and 2012 time periods have been combined to buildan average SED using thefmerge8 HEASARC tool. Flux upperlimits at 95% confidence level were calculated for each time binwhere the test statistic (TS9) value for the source was below 9.

5http://fermi.gsfc.nasa.gov/ssc/6The CLEAN class was chosen in this analysis

since it ensures a higher signal-to-noise ratio with re-spect to the SOURCE class. For more information refer tohttp://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT_caveats_pass7.html.

7http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html8https://heasarc.gsfc.nasa.gov/ftools/caldb/help/fmerge.txt9The Test Statistic value quantifies the probability of having a

pointlike γ-ray source at the location specified. It corresponds roughly

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A&A proofs:manuscript no. aa_1ES1011_arxiv

The systematic uncertainty on the flux is dominated by the sys-tematic uncertainty on the effective area, which is estimated tobe 10% at 100 MeV, decreasing to 5% at 560 MeV, and increas-ing to 10% at 10 GeV (Ackermann et al. 2012). The systematicuncertainties are smaller than the statistical uncertainties of thedata points in the light curves and spectra.

2.3. Swift/XRT and Swift/UVOT

The Swift satellite (Gehrels et al. 2004) performed four obser-vations of 1ES 1011+496 between 2012 March 20 and 31, aspart of a target of opportunity request for a dedicated MWLcampaign. The observations were performed with all three on-board instruments: the X-ray Telescope (XRT;Burrows et al.2005, 0.2–10.0keV), the Ultraviolet Optical Telescope (UVOT;Roming et al. 2005, 170–600nm) and the Burst Alert Telescope(BAT; Barthelmy et al. 2005, 15–150keV). The hard X-ray fluxof this source is below the sensitivity of the BAT instrumentforthe short exposures of these observations, therefore the data fromthis instrument are not used.

The XRT data were processed with standard procedures(xrtpipeline v0.12.6), filtering and screening criteria by us-ing theHeasoft package (v6.13). The data were collected inphoton counting mode, and only XRT event grades 0–12 wereselected (according to the Swift nomenclature;Burrows et al.2005). The XRT observations showed a source count rate>0.5 counts s−1 requiring a pile-up correction. Source events wereextracted from an annular region with an inner radius of 5 pix-els (estimated by means of the PSF fitting technique) and anouter radius of 30 pixels (1 pixel∼2′′.36). Background eventswere extracted within an annular region centered on the sourcewith radii of 70 and 120 pixels. Ancillary response files weregenerated withxrtmkarf, and account for different extractionregions, vignetting and PSF corrections. We used the spectral re-distribution matrix v014 in the Calibration database10 (CALDB20131220) maintained by HEASARC. TheSwift/XRT spectrawere rebinned in order to have at least 20 counts per energybin. Considering the low number of photons collected (< 200counts) the spectrum collected on 2012 March 23 was rebinnedwith a minimum of 1 count per bin and the Cash statistic (Cash1979) was used. A fit was performed with Xspec (v12.7.1)adopting an absorbed power-law model with free photon in-dex using the photoelectric absorption modeltbabs with a neu-tral hydrogen column fixed to its Galactic valueNH = 8.38×1019 cm−2 (Kalberla et al. 2005). During theSwift pointing theUVOT instrument observed 1ES 1011+496 in theV, B, U, andW1, M2 and W2 photometric bands (Poole et al. 2008). Theanalysis was performed using theuvotsource tool to extractcounts from a standard 0′′.5 radius source aperture. To calculatethe source flux, a correction for coincidence losses and a back-ground subtraction was applied. The background counts werederived from a circular region of 10′′radius in the source neigh-borhood. Conversion of magnitudes into dereddened flux den-sities was obtained by adopting the extinction value E(B−V) =0.010 fromSchlafly & Finkbeiner(2011), the mean Galactic ex-tinction curve fromFitzpatrick (1999) and the magnitude-fluxcalibrations byBessell et al.(1998).

to the standard deviation squared assuming one degree of free-dom (Mattox et al. 1996). The TS is defined as−2 log(L0/L), whereL0 is the maximum likelihood value for a model without an additionalsource (i.e. the ’null hypothesis’) andL is the maximum likelihoodvalue for a model with the additional source at the specified location.

10http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/

2.4. KVA and Liverpool telescopes

The optical data were collected with the KVA telescopes11 lo-cated at the Roque de los Muchachos observatory on La Palma.They are operated under the Tuorla Blazar Monitoring Pro-gram12, which runs as a support program to the MAGIC observa-tions. The program started at the end of 2002 and uses the KVAtelescope together with the Tuorla 1 m (located in Finland) tomonitor VHEγ-ray candidates (Costamante & Ghisellini 2002)and known TeV blazars in the optical waveband. It is also usedto alert MAGIC on high states of these objects in order totrigger follow-up VHEγ-ray observations. 1ES 1011+496 wasone of the objects on the original target list and has thereforebeen monitored regularly since the beginning of the program.The data presented here comprise 2011 and 2012 observations.Both KVA telescopes are operated remotely from Finland. Thesmaller of the two telescopes, a 35 cm Celestron, is used forphotometric measurements, while the larger one (60 cm) is usedfor polarimetric observations of some of the brighter objects.The photometric measurements are performed in the optical R-band using differential photometry, i.e., the target and the cal-ibrated comparison stars are recorded on the same CCD im-ages (Fiorucci et al. 1998). The magnitudes of the source andcomparison stars are measured via aperture photometry and areconverted to fluxes applying the formulaF(Jy)= F0 × 10−0.4m,whereF0 is a filter-dependent zero point (F0 = 3080 Jy in theR-band, fromBessell 1979). In order to obtain the AGN coreemission, contributions from the host galaxy and possible nearbystars that add to the overall measured flux have to be subtracted.In the case of 1ES 1011+496, the host galaxy contribution is(0.49± 0.02) mJy (Nilsson et al. 2007).

In 2012 the optical polarimetry data were taken from mid-March to the end of May with the fast readout imaging po-larimeter RINGO 2 (Steele et al. 2010) mounted on the Liver-pool telescope. The instrument is equipped with a hybrid V+Rfilter consisting of a 3 mm Schott GG475 filter cemented to a2 mm KG3 filter. The polarimeter uses a rotating polaroid witha frequency of∼1 Hz that takes eight exposures of the sourceduring a cycle. To determine the degree and angle of polar-ization, these exposures were synchronized with the phase ofthe polaroid (Mundell et al. 2013). The data was analyzed asin Aleksic et al.(2014a) using the standard procedures.

2.5. Metsähovi and OVRO telescopes and VLBA

The 37 GHz observations were performed with the 13.7-mdiameter Metsähovi Radio Telescope,13 a radome-enclosedparaboloid antenna situated in Finland, during the second half ofthe 2012 MWL campaign from mid-March to mid-May. Mea-surements were performed with a 1 GHz-band dual beam re-ceiver centered at 36.8 GHz , whose high electron mobility pseu-domorphic transistor front end operates at room temperature. So-called ON-ON observations were performed during which thesource and the sky are alternated in each feed horn. The fluxdensity scale was set by observations of DR 21 (a huge molec-ular cloud located in the constellation of Cygnus which is usedas a standard candle for radio astronomy), whereas the sourcesNGC 7027, 3C 274 and 3C 84 were used as secondary calibra-tors. A detailed description of the data reduction and analysiscan be found inTeraesranta et al.(1998). The error estimated

11http://www.astro.utu.fi/telescopes/60lapalma.htm12Project web page:http://users.utu.fi/kani/13http://metsahovi.aalto.fi/en/

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Aleksic et al.: MAGIC and multifrequency observations of 1ES 1011+496 in 2011 and 2012

Fig. 1. Daily-binned light curve of the integral VHE-ray emission (red points) from 1ES 1011+496 above 150 GeV during observations carriedout from 2011 to 2012. Upper limits (gray arrows) at 95% confidence level were derived according to theRolke et al.(2005) method for each timebin where the observed integral flux was negative or with flux estimation smaller than its error (red points with dashed error bars). The mean fluxlevel (black dashed line) is retrieved from a fit with a constant to the light curve including the points which are negativeor whose relative error isgreater than 100%.

in the flux density includes the contribution from the measure-ment RMS and the uncertainty of the absolute calibration. Upperlimits at 95% confidence level were calculated for each measure-ment with a signal to noise ratio of S/N < 4.

Regular 15 GHz observations of 1ES 1011+496 were carriedout using the OVRO (Owens Valley Radio Observatory) 40 mtelescope (Richards et al. 2011) that is located in California. Thecenter frequency of the receiver is 15 GHz with a bandwidth of3 GHz. The two sky beams are Dicke switched, and the sourceis alternated between the two beams in an ON-ON fashion to re-move atmospheric and ground contamination. A noise level ofapproximately 3–4 mJy in quadrature with about 2% additionaluncertainty, mostly due to pointing errors, is achieved in a70 sintegration period. Calibration is achieved using a temperature-stable diode noise source to remove receiver gain drifts. Occa-sional gaps in the data sampling are due to poor weather condi-tions or maintenance. The data were calibrated against 3C 286with an assumed flux density of 3.44 Jy at 15 GHz (Baars et al.1977), and analyzed via the pipeline described inRichards et al.(2011). The observations of 1ES 1011+496 were carried out inthe framework of a blazar monitoring program (Richards et al.2011) measuring the source flux density twice a week.

The Very Long Baseline Array (VLBA14) is an interferom-eter consisting of ten identical 25-m antennas on transcontinen-tal baselines up to 8000 km, which are remotely controlled fromthe Science Operations Center in Socorro, New Mexico. Thereceived signals are amplified, digitized, and recorded on fast,high-capacity recorders and are sent from the individual VLBAstations to the correlator in Socorro. Observations are performed

14http://www.vlba.nrao.edu/

at frequencies from 1.2 GHz to 96 GHz in eight discrete bandsand two narrow sub-GHz bands, including the primary spectrallines that produce high-brightness maser emission.

1ES 1011+496 has been monitored with the VLBA in MO-JAVE at 15 GHz since May 2009. MOJAVE15 is a long-termprogram to monitor radio brightness and polarization variationsin jets associated with active galaxies visible in the northernsky (Lister et al. 2009). Seven observations have been performedon 1ES 1011+496 with the 2 cm VLBA from 2009 May to 2012December with a cadence of one to two measurements per year.

3. Results

3.1. MAGIC data

After applying event selection cuts, the stacked analysis fromboth years yields an excess of 1002γ-like events above 100 GeVwithin 0.026deg2 of the distribution of the squared angular dis-tanceθ2 between the reconstructed event direction and the cata-logue position of 1ES 1011+496. The background level of 5242events was estimated applying the same event cuts and using theanti-source position located at 180 with respect to the recon-structed position of the source in the camera as Off region. Wefind a strong signal of∼9.4σ significance, calculated accordingto Li & Ma (1983, eq. 17).

The daily VHEγ-ray light curve above 150 GeV from 2011and 2012 MAGIC observations is shown in Fig.1. The fit ofthe light curve with a constant function gives a probabilityof∼21% (χ2/d.o.f.16= 42/36) for non-variable emission at a mean

15http://www.physics.purdue.edu/astro/MOJAVE/16d.o.f.: Degrees of freedom.

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Year Range f0obs Γobs Γdeabs F (> 200 GeV)[GeV] [10−10 ph cm−2 s−1 TeV−1] [10−11 ph cm−2 s−1]

2007 150− 590 2.0± 0.1 4.0± 0.5 3.9± 0.7 1.58± 0.322008 120− 910 1.8± 0.5 3.3± 0.4 2.2± 0.4 1.3± 0.3

2011/2012 95− 870 1.33± 0.06 3.66± 0.22 3.0± 0.3 0.75± 0.12Table 1. VHE γ-ray spectrum of 1ES 1011+496 observed with MAGIC in 2007 (Albert et al. 2007a), 2008 (Ahnen et al. 2015), and between2011 and 2012. From left to to right: Year of observation, fit range, flux normalizationf0 at 200 GeV, spectral slopesΓobs andΓdeabsfrom a simplepower-law fit of the observed and deabsorbed spectrum using the EBL models fromKneiske et al.(2002) for 2007 and fromDomínguez et al.(2011) for 2008 and 2011/2012 observations, respectively.

Fig. 2. Observed (red filled triangles) VHEγ-ray differential spectrumof 1ES 1011+496 from 2011 and 2012 MAGIC stereo data. The spec-trum is fitted by a simple power law (red solid line) whose parame-ters are indicated in the inlet. For comparison the differential spectra(gray and black circles) from mono observations in 2007 (Albert et al.2007a) and 2008 (Ahnen et al. 2015) and the Crab Nebula spectrum(pink dashed line) are plotted (Aleksic et al. 2015a).

flux level of (1.46± 0.16)× 10−11 ph cm−2 s−1 corresponding to(4.53±0.50)% of the Crab Nebula flux (C.U.). During 2011/2012observations, the integral flux above 200 GeV is lower with re-spect to the flux measured by MAGIC during the source discov-ery epoch in VHEγ-rays (Albert et al. 2007a) and the MWLcampaign in 2008 (Ahnen et al. 2015), when the source was in ahigh state in this energy range (Table1).

The differential spectrum (Fig.2) shows a good agree-ment with a simple power law in the range from∼100 GeV to∼900 GeV. The flux normalizationf0 at 200 GeV is equal to(1.33± 0.06)× 10−10 ph cm−2 s−1 TeV−1, and the photon indexΓ was found to be 3.66±0.22. The spectrum was unfolded usingthe Tikhonov algorithm to correct for the finite energy resolu-tion. Different unfolding algorithms as described inAlbert et al.(2007b) were compared and found to agree within the errors.The systematic uncertainties in the spectral measurementswithMAGIC stereo observations are 11% in the normalization fac-tor (at 300 GeV) and 0.15− 0.20 in the photon index. The erroron the flux does not include the uncertainty on the energy scale.The energy scale of the MAGIC telescopes is determined with aprecision of about 17% at low energies (E < 100 GeV) and 15%at medium energies (E > 300 GeV). Further details are reportedin Aleksic et al.(2012). The observedγ-ray flux was correctedfor absorption by extragalactic background light (EBL) accord-ing to the model ofDomínguez et al.(2011). The deabsorbeddifferential spectrum is in good agreement with a simple powerlaw (χ2/d.o.f.= 2/4, 69% fit probability) that is parametrized by

a photon indexΓ = 3.0 ± 0.3 and a flux normalizationf0 at200 GeV of (1.87± 0.08)× 10−10 ph cm−2 TeV−1.

The energy range of the differential spectrum presented hereis slightly extended to lower energies with respect to previousMAGIC observations (Table1). The measurement of the spec-tral index is consistent with previous observations withinthe er-rors. The results on the normalized differential flux are consis-tent within the systematic errors and the intrinsic spectral slopesfrom a simple power law fit to the deabsorbed spectra show con-sistency within the statistical errors.

3.2. Multiwavelength light curves

In Fig. 3 the stitched 2011 and 2012 MWL light curves are pre-sented. Moreover, we report the long-term behavior of the source(Fig. 4). The intrinsic variability amplitude was quantified withthe fractional variabilityFvar as defined inVaughan et al.(2003).The uncertainty inFvar was computed following the prescrip-tion from Poutanen et al.(2008) as described inAleksic et al.(2015b). The fractional variability at different energies is re-ported in Fig.5 for both the MAGIC 2011/2012 observationsand the long-term datasets. The figure only shows those bandswith positive excess variance (i.e. variance larger than the meanerrors square) because the fractional variability is not defined fornegative excess variances. Such negative excess variancesare in-terpreted as absence of variability, either because there was novariability or because the instruments were not sensitive enoughto detect it.

Possible variations in the source emission in HEγ-raysshown in Fig.3 have been tested following the same likelihoodmethod described in the 2FGL catalogue (Nolan et al. 2012).The method, applied to the 2012 3-day binned light curve indi-cates that the flux is not significantly variable (TSvar = 48 for 49d.o.f.)17. Considering the 2011 and 2012 data samples, the time-averaged integrated flux in theFermi-LAT energy range calcu-lated from 300 MeV to 300 GeV, is (2.4±0.2)×10−8ph cm−2 s−1

with a spectral index of 1.78± 0.05 (TS= 966).TheSwift/XRT data indicate some variability (Fvar = 0.18±

0.05) in X-rays (0.3−10keV), with a mean flux (determinedwith a fit to the data points using a constant) of (4.7 ± 0.1) ×10−12 erg cm−2 s−1. The spectral indices obtained from a simplepower-law fit to the data (Table2) are in agreement within theerrors. The optical and UV bands measured withSwift/UVOTshow a very modest variability (<∼8%) in comparison withfractional variability measured in the R-band (13%). This rel-atively low variability measured withSwift/UVOT could be re-lated to the very limited temporal coverage during the coordi-nated multi-instrument observations in 2011/2012 (see Fig.3).

17If the null hypothesis is correct, i.e. the source flux is constantacross the considered interval, TSvar is distributed asχ2 with 49 degreesof freedom, and a value of TSvar > 74.9 is used to identify variablesources at a 99% confidence level.

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Fig. 3. Stitched 2011 and 2012 MWL light curve of 1ES 1011+496 zoomed into the observation periods from February to April and from Januaryto May. From top to bottom: VHEγ-ray (red circles) and HEγ-ray (orange triangles) data by MAGIC and byFermi-LAT, observations in X-rays(green squares), UV (gray triangles, stars and circles) andoptical U, B and V bands (gray, cyan and magenta squares) bySwift (XRT and UVOT), inthe optical R-band (purple triangles) by the KVA telescope (host galaxy subtracted;Nilsson et al. 2007), optical polarimetry data taken with V+Rfilter by the Liverpool telescope (RINGO2) and radio data provided by the OVRO (blue diamonds) and the Metsähovi telescopes (red crosses).Upper limits of 95% confidence level are indicated by downward arrows (see text for details). The light curves are daily binned except HEγ-rays,where a seven and three day binning was applied to the 2011 and2012 data, respectively. The time axis between 2011 and 2012observation isdiscontinuous.

Previous observations of this object showed a higher R-bandflux(seeAlbert et al. 2007a; Ahnen et al. 2015) and the fractionalvariability of the long-term light curve in this band exceeds 25%.

The radio emission monitored by the Metsähovi (37 GHz)and OVRO (15 GHz) telescopes shows in both cases variabil-ity (Fvar = 0.39± 0.13 and 0.061± 0.006, respectively) withmean flux levels of (0.35± 0.05) Jy and (0.196± 0.027)Jy and achange in flux of 0.23 Jy (77%) and 0.06 Jy (28%), respectively.Given the small statistical errors associated with observationsat 15 GHz, the mean flux level of (0.246± 0.001)Jy appearsslightly lower in 2012 compared to (0.277± 0.001)Jy in 2011.In the case of the OVRO data, the variability was also studiedin Richards et al.(2014), who calculated the intrinsic modulationindex using four years of OVRO data between 2008 and 2012.The intrinsic modulation index (defined as intrinsic standard de-viation over intrinsic mean flux density) describes the variabilityof the source when sampling effects and observational uncer-tainties are accounted for (Richards et al. 2011). The intrinsicmodulation index for 1ES 1011+496 is (0.054± 0.004)Jy, cor-responding to a variability amplitude of 5% indicating modestvariability.

The comparison of the long-term radio light curves compiledfrom OVRO (blue blank diamonds in Fig.4) and MOJAVE ob-servations (black markers) indicates that the decreasing trendof the flux observed by OVRO originates most likely from theradio core (blank black circles), which is following this trend,while the flux emission of the jet components seems to vary ran-domly (filled black symbols). Thus, variability in the radiofluxcan most likely be associated with the radio core. However, thefractional variability amplitude values for the various jet compo-nents indicate that the variability observed in radio couldalso beassociated with the radio jet.

3.3. Long-term correlation studies

We studied the correlations between the light curves in radio,optical R-band and HEγ-rays reported in Fig4. For the ra-dio/optical correlation we used only observations for which thedifference in observation time was less than one day, resulting

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Fig. 4. Long-term MWL light curves of 1ES 1011+496. in the top panel the monthly binned HEγ-ray light curve (orange triangles) from the3FGL (Acero et al. 2015) and daily-binned optical R-band light curve (purple triangles) from the KVA telescope are shown. The radio dataat 15 GHz of the OVRO telescope (blue diamonds) and MOJAVE (black markers) are reported in the lower panels. MOJAVE provides fluxmeasurements of the radio core (blank circles) and the various jet components (C1 to C5; filled symbols).

Fig. 5. Fractional variability amplitude,Fvar as a function of frequency for data simultanous to the MAGIC observation periods (left) shown inFig. 3 and long-term data samples (right) shown in Fig.4. TheFvar of the radio core (right) is marked by a blank circle while thevalues computedfor the components C1 to C5 are represented by filled symbols (C1: circle; C2: upward triangle; C3: star; C4: downward triangle; C5: square).

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Observation Net Exposure Time Photon index Flux 0.3−10 keVa χ2/

[Date] sec Γ [10−13 erg cm−2 s−1] d.o.f.2012-03-20 2285 2.35± 0.06 5.34± 0.22 56/622012-03-23 210 2.50± 0.26 4.55± 0.81 Cashb

2012-03-27 1618 2.12± 0.08 5.67± 0.31 46/392012-03-31 2195 2.33± 0.08 3.50± 0.22 39/35

Table 2. Log and fitting results ofSwift/XRT observations of 1ES 1011+496 in the 0.3− 10 keV band using a power-law model withNH fixed toGalactic absorption.a: Observed flux;b: The Cash statistic (Humphrey et al. 2009) was used to fit the spectrum.

in a sample of 56 data points. Since the HEγ-ray light curve18

is monthly binned, we rebinned the radio and optical data usingthe HEγ-ray light curve bin edges to match the data samples,providing a sample of 45 and 26 points in the case of radio/HEgamma-ray and optical/HE gamma-ray correlation, respectively.

Although the optical light curve seems to show many fea-tures that are uncorrelated to simultaneous radio observations,we find a significant (5.4σ) linear (Pearson) correlation of0.63+0.08

−0.09 strength between radio and optical, which is driven bythe decrease in the radio and optical flux around MJD 55700. Nosignificant linear correlation was found between the optical bandand HEγ-rays and radio frequencies and HEγ-rays.

3.4. Optical and radio polarimetry

The optical polarimetry data display a very low degree of opticalpolarization (P) with a mean value of 2.5 ± 0.6% (Fig.3). Theepochs of optical polarimetry measurements coincide with thosewhen the photometric KVA data exhibit smooth low-amplitudeoscillations in the total flux, but no correlation is observed. Infact, no significant variability is detected in P, and the statisticalerrors of the low-level polarization measurements are dominat-ing. As for electric vector position angle (EVPA), it shows agen-eral trend throughout the observation period by which the anglesteadily decreases from roughly−50 to about−100.

Figure6 shows the multi-epoch 15 GHz radio map of thesource provided by MOJAVE. The radio morphology consistsof a compact optically thick core, and more diffuse jet emissionthat extends to the west. In the observed dates, from May 2009to December 2012, the jet position angle (PA) is stable, orientedat−100 to −80, approximately. This is compatible with previ-ous measurements byAugusto et al.(1998) andNakagawa et al.(2005), who reported values of−99 and−105, respectively, atthis frequency.

The EVPA in radio behaves differently at earlier and laterepochs: before 2011, the core EVPA is decreasing from∼-45 to−15, whereas the jet EVPA is rather stable at roughly -25 (Ta-ble3). In 2011 the core EVPA is about−160, the electric vectorhaving moved in the clockwise direction from its original posi-tion, at a final angle of roughly−120; the jet EVPA remainedconstant during all epochs, at about−15 to −40.

But the most interesting feature about the jet polarizationin radio is a relatively large activity in the amount of frac-tional linear polarization seen, with some bright featuresappear-ing at different times and positions within the jet which havea degree of fractional linear polarization up to 60%, close tothe maximum value expected from homogeneous synchrotronsources (Pacholczyk 1970). These values of fractional linear po-larization are much higher than what is seen in the optical, andin fact appear to bear little resemblance to the general state of

18Data taken from the 3FGL (Acero et al. 2015), available athttp://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermilpsc.html.

Fig. 6. MOJAVE 15 GHz VLBA images of 1ES 1011+496 at sevenepochs from 2009 to 2012. The left hand images show total intensitycontours, with electric polarization vectors overlaid in blue. The righthand images show total intensity contours with fractional linear polar-ization in color ranging from 0 to 0.6. The images have been convolvedwith the same Gaussian restoring beam having dimensions 0.83 ×0.63 mas and position angle−5. In all images, the contour levels arefactor of 2 multiples of the base contour level of 0.9 mJy beam−1. Thepolarization vectors have a scaling of 2 mJy beam−1 mas−1 and are indi-cated for regions with polarized flux density exceeding 0.8 mJy beam−1.The angular scale of the images is 3.4 pcmas−1.

the source polarization at these higher frequencies. The values ofthe fractional linear polarization reported in Table3 are averaged

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Observation EVPACore EVPAJet pCore pJetDate [] [ ] [%] [%]

2009-05-02 −46 −25 1 62009-08-19 −37 −29 3 122010-03-10 −19 −25 2 112010-09-29 −15 −24 2 182011-04-11 −163 −16 1 112012-09-27 −126 −41 3 82012-12-23 −123 −27 2 7

Table 3. EVPA and mean fractional linear polarization of the radio coreand jet at 15 GHz from 7 epochs of MOJAVE observations. The EVPAaccuracy is roughly±5. For the jet the values of the fractional linearpolarization are averaged over the whole jet excluding the core. Thuslocalized regions in Fig.6 have higher and lower fractional polarizationvalues with respect to those reported here.

over the whole jet excluding the core. Thus localized regions inFig. 6 have both higher and lower fractional polarization values.

The relation between the optical and radio EVPA is furthercomplicated by the fact that the optical EVPA follows a counter-clockwise rotation trend throughout the year 2012, when opticalpolarization data was taken, going from 150 to 100 (or equiv-alently−30 to −80 if we allow for the 180 ambiguity in theEVPA definition). This trend is opposite to that followed by theradio core EVPA in 2012 and therefore appears to dissociate theoptical polarized emission, or at least the bulk of it, from what ishappening at the radio core. But when we look at the jet EVPAin radio an accordance is found with the behavior seen in opti-cal. According to Table3, in the last two epochs of radio data,the overall radio jet EVPA was pointing between−15 and−40.The direction is off by quite a few degrees, but nevertheless simi-lar to what is established in optical. Furthermore the trendis alsocounter-clockwise.

Although optical and radio jet polarized emission cannot beconfidently associated on this basis alone, one has to keep inmind that from the radio maps, the jet structure is quite com-plex, with bright features characterized by quite high polarizedemission levels. Likewise, the polarization vectors that are asso-ciated with these individual regions do not behave all the sameor have the same orientations. Based on that we could speculatethat one or more of these bright features seen in radio are alsothe zones responsible for the bulk of the optical polarized emis-sion – as would be logical to expect – but in optical, differentlyfrom radio, the absence of good-enough spatial resolution pre-vents one from getting a clear picture. In fact, the poor spatialresolution would have the effect of lowering the net polariza-tion of the source, as regions with slightly different polarizationdirections are seen superposed and the net effect of a preferen-tial direction of the field is washed away. Nevertheless, thefactthat we see a broad orientation for the optical EVPA towards thesame rough direction of the radio jet EVPA, and that the trendof rotation of both also matches, could be taken as an indicationthat the optical emission is also produced in the bright featuresof the jet. If these are zones of particle acceleration, for exampleshocked plasma zones where the field intensity and degree of or-dering is also enhanced, then this would provide provide someinsight on the nevertheless complex dynamics of the source.

3.5. Jet kinematics

Based on the first five epochs presented in Fig.6, a statisti-cally significant (≥ 3σ) expansion rate of 131± 27µas yr−1 cor-

responding to an apparent speed of 1.8 ± 0.4c was found forthe bright jet feature at 2 mas from the core (Lister et al. 2013).The last epoch has poor data quality due to three VLBA an-tenna drop-outs. No other components display motion at suchstatistical significance. Out of the 45 known TeV HBLs19, 13have been targets of VLBA measurements (Lister et al. 2013;Piner & Edwards 2013; Tiet et al. 2012; Piner et al. 2010). Themajority of these HBLs show rather low apparent speeds,i.e. < 1c. Beside 1ES 1011+496, a superluminal mo-tion (e.g. Urry & Padovani 1995; Ghisellini 2000) of 1.2 ±0.4c (Piner et al. 2010) was measured for the HBL H 1426+428with a statistical significance of≥ 2σ. Given the statistical er-ror, the apparent speed of the latter could also be< 1c, whichmakes 1ES 1011+496 the HBL with the highest statistically sig-nificant superluminal speed measured so far. However, sincethemeasured apparent speed for this source is still compatiblewiththe speed of light within 2σ, a highly significant detection ofsuperluminal motion in a TeV HBL can not be claimed yet.

4. Modelling the SED

Due to the general low state of the source in the observed energybands in 2011 and 2012, the data have been combined to an aver-age SED (Fig.7), except X-ray observations, where the highest(2012 March 27) and lowest (2012 March 31) flux observed arereported instead. Corrections for EBL absorption were applied tothe VHEγ-ray data according to the model byDomínguez et al.(2011), while the data fromSwift in the UV bands and opticaldata in the R-band from the KVA telescope were corrected forGalactic extinction (Fitzpatrick 1999) and host galaxy contri-bution (Nilsson et al. 2007), respectively. For comparison, weshow archival data available at the ASI Science Data Center(ASDC)20. Both the low and high-energy bump of the SED arewell constrained by these simultaneous MWL data. For the lat-ter, a connection of the VHE and HEγ-ray band was achievedfor the first time for 1ES 1011+496. The SSC model used to de-scribe the data locates the peak of the inverse Compton bump ataround 20 GeV.

The SED shows no indication for the previous hypothesisof an inverse Compton dominance (Albert et al. 2007a). Thisprevious assumption is likely related to missing complementaryMWL data, whereby both peaks were barely constrained. Fromthe SED presented here, the maximum fluxνFν of both energybumps seems to be nearly equal (2.75×10−11erg cm−2 s−1 and2.26×10−11erg cm−2 s−1 for the synchrotron and inverse Comp-ton peak, respectively).

A one-zone synchrotron-self-Compton (SSC)model (Maraschi & Tavecchio 2003) was applied to repro-duce the broadband SED, assuming a spherical emission regionof radiusR filled with a tangled magnetic field strengthB. Aprimary spectrum of a relativistic electron population is approx-imated by a smoothed, broken power law that is parametrizedby the minimum (γmin), break (γb) and maximum (γmax) Lorentzfactors, the slopes before (n1) and after (n2) the break and theelectron density parameterK. Relativistic effects are taken intoaccount by the Doppler factorδ. Absorption ofγ-rays in theemitting region by photon-photon pair production on internalsoft (e.g. synchrotron) photons (e.g.Dondi & Ghisellini 1995)is self-consistently accounted for in the model, but negligible(τ ≪ 1) for the current set of parameters. The emission isself-absorbed at radio frequencies, implying that it is dominated

19http://tevcat.uchicago.edu; current catalog version: 3.40020http://www.asdc.asi.it/

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Year γmin γb γmax n1 n2 B K R δ

[103] [104] [105] [G] [103 cm−3] [1016 cm]2007a 3.0 5.0 200 2.0 5.0 0.15 20 1.0 202008b 7.0 3.4 8.0 1.9 3.3 (3.5) 0.048 0.7 (0.8) 3.25 26

2011/2012I 10.0 4.0 7.0 2.0 3.7 0.19 10.0 1.0 202011/2012II 10.0 3.3 4.0 2.0 3.8 0.19 13.4 0.9 20

Table 4. Input model parameters assumed for the SSC model (Maraschi & Tavecchio 2003) shown in Fig.7. The model parameters for theSED modeling of previous observations are reported for comparison. I : X-ray spectrum from March 27;II : X-ray spectrum from March 31;a: Albert et al.(2007a); b: Ahnen et al.(2015). The model parameters reported for the modeling of the 2008data consider the high (low) stateobserved in X-rays, while those listed for the modeling of the 2007 data are based on the MAGIC spectrum that has been corrected for EBLabsorption using the model byKneiske et al.(2002) current at that time.

Fig. 7. Averaged SED of 1ES 1011+496 compiled from simultane-ous 2011 and 2012 MWL observations marked in red. We combinedeabsorbed (Domínguez et al. 2011) VHE γ-ray observations (circles)by MAGIC and HEγ-ray data (triangles) fromFermi-LAT, Swift datafrom 2012 March 27 (filled squares) and 31 (blank squares) in X-raysand UVOT bands (squares), the latter corrected for Galacticextinc-tion (Fitzpatrick 1999), optical data in the R-band (star) from KVA (cor-rected for host galaxy contribution;Nilsson et al. 2007) and radio dataat 15 GHz (diamond) and 37 GHz (cross) provided by the OVRO andMetsähovi telescopes. The solid (dashed) line represents the fit with aone-zone SSC model considering the X-ray spectrum from March 27(March 31). The parameters are reported in Table4. Previous MAGICobservations carried out in 2007 (black diamonds;Albert et al. 2007a)and 2008 (gray circles;Ahnen et al. 2015) are corrected for EBL ab-sorption according to the model byDomínguez et al.(2011). The insetis a zoom into the HE to VHEγ-ray band. Archival data (gray squares)are taken from the ASDC20.

by the outer regions of the jet. Therefore, radio data are notincluded in the SED modeling. However, the predicted radioflux of the emission region does not violate the observed one,showing variations over half-year long timescales (Fig.4),which hint to emission regions that are likely associated toscales larger than those commonly considered for the high-energy emission in this kind of sources. The parameters of theone-zone SSC model can be uniquely fixed once the SED peaks

(frequencies and luminosities) and the variability timescalesare known (Tavecchio et al. 1998). The physical parametersassumed for this model are listed in Table4 together withthose derived from 2007 and 2008 observations using the samemodel. In the present case we do not have any estimate of thevariability timescale, which is directly linked to the source size,and thus the set of parameters cannot be fully constrained. Wethus assume a radius of the emitting region and a Doppler factorclose toR ≈ 1016 cm andδ = 20, values commonly found inthis kind of sources (e.g.Tavecchio et al. 2010; Aleksic et al.2014b, 2015c,d). The other parameters derived by reproduc-ing the SED are also similar to those typically inferred forHBLs (Tavecchio et al. 2010). In particular the low magneticfield strength is quite common for HBLs (e.g.Finke et al. 2008;Dermer et al. 2015) rather than being typical for IBLs, leadingto deviations from equipartition.

The cooling time for the electrons emitting at the synchrotronpeak (considering both synchrotron and inverse Compton losses)tcool is 2.7× 105 s is quite close to the escape timetesc ∼ R/c =3 × 105 s as suggested byTavecchio et al.(1998). The energydensity of the electrons and the magnetic fieldUe andUB cor-respond to 7.3 × 10−2 and 1.4 × 10−3 erg cm−3 indicating themagnetic field to be much below equipartition,UB/Ue = 0.02.A quite general result in the framework of the one-zone SSCmodel for TeV emitting BL Lacs is the large ratioUe/UB, indi-cating that the particle energy density is largely dominating overthe magnetic one. This is quite a robust result and representsa problem for both jet theory and particle acceleration model(e.g.Tavecchio & Ghisellini 2016, and references therein). Pos-sible solutions include inhomogeneous models such as the so-called structured jet model. In this specific case, the jet isthoughtto be composed by a fast spine, which is responsible for the emis-sion observed from blazars, surrounded by a slower sheath. Thelarge photon energy density in the emitting region, provided bythe sheath, allows one to increase the magnetic energy density inthe spine, thus decreasing theUe/UB ratio required to reproducethe observed SED.

As for other TeV HBLs (Piner & Edwards 2013), the Lorentzfactor derived from the modeling of the SED is larger than thatinferred for the superluminal speed measured in the radio band.A possibility to solve the problem is that the jet decelerates fromthe innermost blazar region to the outer regions responsible forthe radio emission (e.g.Georganopoulos & Kazanas 2003), orthat the radio and TeV emission derive from separate regions,the former being produced in a slow layer surrounding a fast,TeV emitting spine (Ghisellini et al. 2005).

The comparison with previous models of the source SED(Albert et al. 2007a; Ahnen et al. 2015) indicates a good agree-ment for most of the parameters. The radius of the emitting re-gion derived inAhnen et al.(2015) is about a factor of 3 larger

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than in the other cases. The minimum and maximum Lorentzfactors show relatively large variations among the models butthese parameters are usually not well constrained by the avail-able data. The parameters from the 2008 modeling are in goodagreement with the model presented here. Most likely, variationsamong the individual parameters are rather related to the previ-ously poor MWL coverage than to important variations of thephysical processes operating in 1ES 1011+496.

5. Conclusion

While the time-averaged VHE spectrum observed in 2011 and2012 is consistent in spectral slope with MAGIC observationsfrom 2007 and 2008, the integral flux above 200 GeV is lowerwith respect to previous VHE observation epochs. The deab-sorbed VHEγ-ray spectrum, for which EBL corrections wereapplied, is in good agreement with a power law, with a spectralindex that is consistent within the statistical errors withpreviousmeasurements of this parameter.

The MWL data of 1ES 1011+496 from 2011 and 2012 indi-cated a general low state of the source across the electromagneticspectrum. We did not find statistically significant variability inVHE and HEγ-rays, while in the R-band the source varied no-tably without undergoing any major flare. The flux in the UV andU bands showed a decreasing trend; however due to the smallobservation window in X-rays and the UVOT bands, no clearconclusion can be drawn on the variability in these wavebands.Small variability was found at 15 GHz, while a hint for moder-ate variability seemed to be on the signal at 37 GHz that can mostlikely be associated with the radio core. Studies of the long-termlight curves showed a significant linear connection betweenop-tical and radio indicating a correlated variability between thesefrequencies. The study of the optical and radio light curveswiththe HEγ-ray Fermi light curve did not show any significant lin-ear correlation.

The source was observed in optical and radio polarization atseveral epochs since 2009. VLBI data from 2009 to 2010 theEVPA of the radio jet was rather constant aligned at around−25, while the EVPA of the radio core decreased from about−45 to roughly−15. In 2011 the EVPA of the core underwent arotation, arriving at a final angle of about−125, rotated nearly100 in the clockwise direction from its initial value. In the jet,features with very high values of polarization up to 60% wereobserved. These polarization features seem not to contribute toomuch to to the optical polarization emission, or are largelydi-luted by non-polarized emission, as the optical degree of polar-ization is very low (< 5%) and rather constant throughout thecampaign. That said, a trend of slow counter-clockwise rotationwas observed in the optical EVPA in 2012, in the same directionfrom certain components of the jet at the latest VLBI epochs.This similarly concurrent trend of EVPA rotation in opticalandradio frequencies suggests that at least part of the opticalemis-sion has origin in some of the bright radio features as detectedby the VLBI observations. A contribution to the optical emissionfrom other parts of the jet with different orientations of the mag-netic field could also explain both the low level of polarizationfrom the unresolved optical source and the non-exact alignmentbetween any of the radio components and the optical EVPAs.In addition, we reported a detection of superluminal motionof1.8± 0.4c in 1ES 1011+496, which is the highest speed statisti-cally significant (≥ 3σ) measured so far in a TeV HBL.

The one-zone SSC model could reproduce the broadbandSED of 1ES 1011+496, which was derived from simultaneous2011 and 2012 MWL data, with parameters similar to those typ-

ically inferred for other HBL objects. From the SED presentedhere, the flux of both energy bumps seems to be nearly equal, be-ing a typical HBL characteristic. The position of the synchrotronpeak of the averaged 2011/2012 SED during the generally lowemission state also favors an HBL nature of the source. TheLorentz factor derived from the modeling of the SED is largerthan that inferred for the superluminal speed measured in the ra-dio band, which could be explained by a deceleration of the jetfrom the innermost blazar region to the outer regions responsi-ble for the radio emission. Another explanation could be that theradio and TeV emission originate from separate regions, wherethe former is produced in a slow layer surrounding a fast, TeVemitting spine. In general, the model parameters are in goodagreement with those adopted for the SEDs from 2007 and 2008MWL observations. Thanks to the connection of the VHE andHE energy band jointly observed for the first time for this source,the frequency of the IC peak was well constrained. The SSCmodel describing the SED located the peak of the inverse Comp-ton bump at∼20 GeV. In the VHE range, an extension to lowerenergies was reached in these new observations.

Acknowledgements. We would like to thank the Instituto de Astrofísica de Ca-narias for the excellent working conditions at the Observatorio del Roque delos Muchachos in La Palma. The financial support of the GermanBMBF andMPG, the Italian INFN and INAF, the Swiss National Fund SNF, the ERDF un-der the Spanish MINECO (FPA2012-39502), and the Japanese JSPS and MEXTis gratefully acknowledged. This work was also supported bythe Centro de Ex-celencia Severo Ochoa SEV-2012-0234, CPAN CSD2007-00042,and MultiDarkCSD2009-00064 projects of the Spanish Consolider-Ingenio2010 programme,by grant 268740 of the Academy of Finland, by the Croatian Science Foundation(HrZZ) Project 09/176 and the University of Rijeka Project 13.12.1.3.02, by theDFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by thePolish MNiSzW grant 745/N-HESS-MAGIC/2010/0.The Fermi LAT Collaboration acknowledges generous ongoing support from anumber of agencies and institutes that have supported both the development andthe operation of the LAT as well as scientific data analysis. These include theNational Aeronautics and Space Administration and the Department of Energyin the United States, the Commissariat à l’Energie Atomiqueand the Centre Na-tional de la Recherche Scientifique/ Institut National de Physique Nucléaire et dePhysique des Particules in France, the Agenzia Spaziale Italiana and the IstitutoNazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports,Science and Technology (MEXT), High Energy Accelerator Research Organiza-tion (KEK) and Japan Aerospace Exploration Agency (JAXA) inJapan, and theK. A. Wallenberg Foundation, the Swedish Research Council and the SwedishNational Space Board in Sweden. Additional support for science analysis duringthe operations phase is gratefully acknowledged from the Istituto Nazionale diAstrofisica in Italy and the Centre National d’Études Spatiales in France.The Metsähovi team acknowledges the support from the Academy of Finland toour observing projects (numbers 212656, 210338, 121148, and others).The OVRO 40-m monitoring program is supported in part by NASAgrantsNNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911.The National Radio Astronomy Observatory is a facility of the National ScienceFoundation operated under cooperative agreement by Associated Universities,Inc. This work made use of the Swinburne University of Technology softwarecorrelator (Deller et al. 2011), developed as part of the Australian Major Na-tional Research Facilities Programme and operated under licence.The MOJAVE project is supported under NASA-Fermi grants NNX12A087G.Part of this work is based on archival data provided by the ASIASDC.

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A&A proofs:manuscript no. aa_1ES1011_arxiv

1 IFAE, Campus UAB, E-08193 Bellaterra, Spain2 Università di Udine, and INFN Trieste, I-33100 Udine, Italy3 INAF National Institute for Astrophysics, I-00136 Rome, Italy4 Università di Siena, and INFN Pisa, I-53100 Siena, Italy5 Croatian MAGIC Consortium, Rudjer Boskovic Institute, University

of Rijeka and University of Split, HR-10000 Zagreb, Croatia6 Max-Planck-Institut für Physik, D-80805 München, Germany7 Universidad Complutense, E-28040 Madrid, Spain8 Inst. de Astrofísica de Canarias, E-38200 La Laguna, Tenerife,

Spain9 University of Łódz, PL-90236 Lodz, Poland

10 Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen,Germany

11 ETH Zurich, CH-8093 Zurich, Switzerland12 Universität Würzburg, D-97074 Würzburg, Germany13 Centro de Investigaciones Energéticas, Medioambientalesy Tec-

nológicas, E-28040 Madrid, Spain14 Institute of Space Sciences, E-08193 Barcelona, Spain15 Università di Padova and INFN, I-35131 Padova, Italy16 Technische Universität Dortmund, D-44221 Dortmund, Germany17 Unitat de Física de les Radiacions, Departament de Física, and

CERES-IEEC, Universitat Autònoma de Barcelona, E-08193 Bel-laterra, Spain

18 Universitat de Barcelona, ICC, IEEC-UB, E-08028 Barcelona,Spain

19 Japanese MAGIC Consortium, KEK, Department of Physics andHakubi Center, Kyoto University, Tokai University, The Universityof Tokushima, ICRR, The University of Tokyo, Japan

20 Finnish MAGIC Consortium, Tuorla Observatory, UniversityofTurku and Department of Physics, University of Oulu, Finland

21 Inst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria22 Università di Pisa, and INFN Pisa, I-56126 Pisa, Italy23 ICREA and Institute of Space Sciences, E-08193 Barcelona, Spain24 Università dell’Insubria and INFN Milano Bicocca, Como, I-22100

Como, Italy25 now at NASA Goddard Space Flight Center, Greenbelt, MD 20771,

USA and Department of Physics and Department of Astronomy,University of Maryland, College Park, MD 20742, USA

26 now at Ecole polytechnique fédérale de Lausanne (EPFL), Lau-sanne, Switzerland

27 now at Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria

28 now at Finnish Centre for Astronomy with ESO (FINCA), Turku,Finland

29 also at INAF-Trieste30 also at ISDC - Science Data Center for Astrophysics, 1290, Versoix

(Geneva)31 now at Centro Brasileiro de Pesquisas Físicas (CBPF\MCTI), R. Dr.

Xavier Sigaud, 150 - Urca, Rio de Janeiro - RJ, 22290-180, Brazil32 INAF-IRA Bologna, Via Gobetti 101, I-40129, Bologna, Italy33 Aalto University Metsähovi Radio Observatory Kylmälä, Finland34 Aalto University Dept of Radio Science and Engineering, Espoo,

Finland35 Cahill Center of Astronomy and Astrophysics, California Institute

of Technology, 1200 E California Blvd, Pasadena, CA91125, USA36 Astro Space Center of Lebedev Physical Institute, Profsoyuznaya

84/32, 117997 Moscow, Russia37 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69,53121

Bonn, Germany38 Department of Physics, Purdue University, 525 Northwestern Av-

enue, West Lafayette, IN 47907, USA39 National Radio Astronomy Observatory, PO Box 0, Socorro, NM

87801, USA40 Astrophysics Research Institute, Liverpool John Moore University,

UK41 Pulkovo Observatory, Pulkovskoe Chaussee 65/1, 196140 St. Peters-

burg, Russia42 Crimean Astrophysical Observatory, 98409 Nauchny, Crimea,

Ukraine43 Laboratoire d’Annecy-le-Vieux de Physique des Particules, Univer-

sitè Savoie Mont-Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux,

France* corresponding authors: C. Schultz, email: cor-nelia.schultz@pd.infn.it, U. Barres de Almeida, email:ulisses@cbpf.br, S. Paiano, email: simona.paiano@pd.infn.it

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